The accuracy and range of analysis of capillary supercritical fluid chromatography (SFC) is affected by the type of injector utilized and the method for using the same. A high performance liquid chromatographic (HPLC) injector having a sample splitter is the most frequently used injector for capillary SFC. While this type of HPLC injector is useful for many applications, there are a number of drawbacks associated with the injector and the current methods for using the injector.
One drawback is that the samples must be relatively concentrated, which can be difficult to obtain for trace analytes or for samples having limited solubilities in appropriate solvents The use of a sample splitter presents the problems of: (1) splitter discrimination; (2) plugging due to the fact that the sample injector uses small sample loops which are easily plugged; and (3) a relatively large volume of the sample is needed for flushing and filling the sample loop prior to the injection.
Splitless injection techniques using HPLC-type injectors with a small sample loop--with a typical volume of 60nL--or very rapid injection techniques have also been described and used with capillary SFC. The use of splitless injection techniques are generally limited to small injection volumes and can result in unacceptably broad solvent peaks. The use, however, of post-injection solvent venting can dramatically reduce solvent tailing.
Similar to capillary gas chromatography (GC), on-column injections are expected to eliminate the possibility of splitter discrimination and to improve the ability of SFC to analyze more dilute samples while requiring less total volume for sample injection by eliminating the need to flush a sample loop. The construction of an on-column injector is far more complicated for SFC than for GC, both because capillary SFC columns have typical inner diameters of only 50 to 100 .mu.m, and because higher pressures are used in SFC.
Early attempts to use on-column injections have involved large diameter (300 .mu.m i.d.) columns or dipping the column into the sample solution The present invention is directed to a simple modification which can be made to a commercially available four-port valve that allows injection volumes as large as .5.mu.L to be placed inside a short retention gap at the head of a 50 .mu.m i.d. capillary SFC column. Also disclosed herein is a method for using the present invention which permits solvent venting with the modified apparatus, which is accomplished in a few simple steps.
The development of the injector and the development of the method for solvent venting required that a number of special concerns be addressed. One concern was that, because the chromatographic oven was held at temperature (typically 125.degree. C.), the column's stationary phase could be exposed to atmospheric oxygen and result in degradation of the column's performance. Another concern was the possible loss of volatile analytes during the solvent venting step. Also, because the sample is injected into a retention gap with a relatively large volume of solvent, refocusing of the analytes in the chromatographic stationary phase was thought to be necessary in order to obtain good chromatographic peak shapes.
The present invention does not require the use of a sample loop, and the injection is performed with a conventional GC on-column syringe. The use of a GC on-column syringe for injection reduces sample waste compared to the injection techniques that utilize sample loops. Additionally, the present invention facilitates the use of solvent venting techniques. The use of solvent venting eliminates solvent tailing and yields chromatographic base lines comparable to those achieved using conventional split injection. Also, the use of solvent venting reduces the requirement for flushing the column between analytical runs and thus reduces the wait for the column to return to a condition suitable to begin the next analysis.
It is therefore a principal object of the present invention to provide an improved injector for use with capillary SFC analysis.
Another object of the present invention is to provide an injector which permits the use of less concentrated samples having trace analytes.
Still another object is to provide an injector which permits the use of samples having limited solubilities in appropriate solvents.
A further object of the present invention is to provide an injector which eliminates splitter discrimination and which eliminates the use of small sample loops.
Yet another object is to provide an injector which is not subject to plugging.
Another object of the present invention is to permit almost on-column injection of a sample into a capillary SFC column.
Another general object of the present invention is to provide a method for using an injector which eliminates solvent tailing.
These and other objects will become apparent to those of ordinary skill in the art.